The telecommunications industry has developed a new international standard called Asynchronous Transfer Mode (ATM) that will be the basis of the forthcoming Broadband Integrated Services Digital Network (BISDN). The ATM standard allows transmission of intermixed audio, video, and data over high-speed links. As well as being used in wide-area networks, the ATM standard can be used for local-area networks to support multimedia applications.
The unit of transmission used in the ATM standard is a cell. An ATM cell contains 53 bytes of information and has the basic format shown in FIG. 1. The four-byte header field contains virtual path and virtual channel identifiers which are used for routing the cell through the ATM network. The HEC byte contains a CRC (cyclic redundancy check) value to check the correctness of the cell header and to allow correction of single-bit errors if desired.
The ATM header uniquely determines the parameters associated with a given connection. Within an end user interface such as a computer multiple connections can be going on at the same time; therefore, there is a need to dispatch these cells to their correct destination based on a given state of the connection, as established during call setup.
Within a switch each ATM cell is switched based on the information contained in its header; more specifically based on its Virtual Path Identifier (VPI) and its Virtual Channel Identifier (VCI) shown in FIG. 1. A combination of VCI and VPI bits are used to index lookup tables that contain the routing information.
In order to support all possible header entries at any network interface a very large table (i.e. 2 28 or about 64 million entries) would be required. However, in most cases, only a few VCI entries are defined for a given VPI and direct addressing is therefore quite wasteful of memory space. Coupled with the fact that a switch supports many input ports, each one with access to its own dedicated address space, there would be required unpractical large size memories to support every possible correction. Existing implementations typically require to `bit-stip` (or ignore some of the bits in order to reduced the table size) some of the VCI or VPI fields in order to reduce table sizes. (T1S1 Technical Sub-Committee. Broadband Aspects of ISDN, Baseline Document, August 1990. Chief Editor: Rajeev Sinha, incorporated by reference).